Key messages

Three of 13 studies from the USA found higher productivity from nests protected by individual barriers than unprotected nests. Two studies from the USA and Sweden found no higher productivity from protected nests.

Eight studies from the USA and Europe found higher hatching rates, or survival, or low predation of nests protected by individual barriers, although two of these found that higher hatching rates did not result in higher productivity. Two small studies from North America found no differences in predation or survival rates between protected and unprotected nests.

A meta-analysis from the USA found that there were differences in the effectiveness of different exclosure designs.

Background information and definitions

If fencing does not work to exclude predators (for example, predatory birds), or is not a viable option, it may be possible to protect individual nests using a variety of cages and exclosures. These must be able to allow chicks and adults to get in and out, but not predators and should be quick to install to minimise the chances of parents abandoning nests (see ‘Can nest protection increase nest abandonment?’).

Unfortunately, because each cage is over a nest, it is possible that predators will learn the association and that providing the exclosures will actually increase predation on either adults or chicks (see ‘Can nest protection increase predation of adult and chick waders?’)

Supporting evidence from individual studies

1

A small randomised, replicated and controlled trial in 1978 on a beach on Lake Erie, Canada (Nol & Brooks 1982) found that predation rates of killdeer Charadrius vociferous nests were not significantly different between 12 nests protected with a novel predator exclosure (‘H’ shaped frame with the nest in the centre and eight 7 x 12 cm openings, covered in 1.4 cm mesh hardware cloth) and 17 control nests (75% of protected nests and 71% of unprotected nests predated). However, no protected nests were lost to avian predators (gulls Larus spp. or American crow Corvus brachyrhynchos). Instead they were predated by raccoons Procyon lotor or mustelids which could enter exclosures.

2

A 1992 meta-analysis (Deblinger et al. 1992) analysed data from 211 nest exclosures across eight US states and three Canadian provinces to determine exclosure features that led to lowest predation rates of piping plover Charadrius melodus nests. Overall, exclosures were effective (10% (21) of nests being predated, mainly by red foxes Vulpes vulpes but also American crows Corvus brachyrhynchos and other predators). Estimatedpredation probabilities revealed that: mid-sized exclosures (3-6 m2) suffered higher predation (26% of 48 exclosures) than small (<3 m2, 5% of 23) or large (>6 m2, 8% of 140) exclosures. Square enclosures were predated at a higher rate (72% of 19) than circular (8% of 166) or triangular (0% of 26) ones. Exclosures supported by ‘tomato stakes’ (thin gardening stakes) were predated more (80% predation of 18 exclosures) than unsupported (3% of 35) or metal/wood supported (8% of 158). Exclosures with mid-height posts (122 cm) were predated more (29% of 40) than short (<122 cm, 3% of 41) or tall (>122, 9% of 130) posts and exclosure with low fences (<122 cm, 42% of 27) were more likely to be predated than those with high (>122 cm, 8% of 184) fences. Fences buried to less than 10 cm were more likely (27% of 62) to be predated than those buried to more than 10 cm (6% of 149). There were no significant differences between different mesh sizes (5 x 10 cm vs. 5 x 5 cm) or whether exclosures were covered or not.

A controlled, replicated study from 1986-1989 on a beach in Massachusetts, USA (Rimmer & Deblinger 1992) found that hatching rates of 26 piping plover Charadrius melodus nests protected by triangular wire mesh fences (5 cm wire mesh, 30.5 m perimeter, placed around individual nests) were higher than for unprotected nests (92% and 25% of nests hatching at least one egg, respectively). On average, protected nests produced significantly more nestlings than unprotected nests (3.5 and 1 chicks/nest respectively). All but one of the losses of unprotected nests was due to predation; no protected nests were predated.

5

A replicated, controlled study in 1992 in the North Slope of Alaska, USA (Estelle et al.1996) found that the average daily survival rate of 13 pectoral sandpiper Calidris melanotos nests protected by wire mesh cages was significantly higher than that for 39 unprotected nests (survival rates of 0.98 for caged and 0.72 for uncaged nests). The mesh of the 31 cm tall and 66-69 cm diameter cages was sufficiently large (5 x 10 cm) to allow female plovers to enter and exit, but prevented arctic foxes Alopex lagopus from digging under or entering nests. No protected nests were lost to predation.

A replicated, controlled study from 1996-1997 at three alkali lakes in North Dakota and Montana, USA (Murphy et al. 2003) found that the average number of fledglings produced by piping plover Charadrius melodus pairs provided with fences (0.9 m tall and made from 5 cm poultry wire) around individual nests was significantly higher than for unprotected pairs (1.7 chicks/pair for 46 pairs with nest fences vs. 0.7 chicks/pair for 43 unprotected nests). This study is described further in ‘Use multiple barriers to protect nests’.

8

A replicated before-and-after study from 1984-90 and 1991-99 onbeaches in California, USA (Neuman et al. 2004) found that the mean hatching rate and hatching rate/male of snowy plover Charadrius alexandrinus nests increased following the protection of nests with 1.5 m high wire exclosures (hatching rate in 1984-90: 43% of 728 nests vs. 68% of 682 nests in 1991-99; hatching rate/male: 2 chicks/male vs. 2.7 chicks/male). However, the mean number of chicks fledged per male did not change (0.86 and 0.81 chicks fledged/male for 1984-90 and 1991-99 respectively). Between 1993 and 1999, there was also the targeted removal of red foxes Vulpes vulpes and feral cats Felis catus, described in ‘Control predators not on islands’. The study also discusses nest abandonment and adult mortality, see ‘Can nest protection increase nest abandonment?’ and ‘Can nest protection increase predation of adults and chicks?’.

9

A small, replicated, before-and-after study at a site in eastern England (Gulickx & Kemp 2007) found that the average productivity of little ringed plover Charadrius dubius increased from 0.6 chicks/pair during 1984–95 to 1.6 chicks/pair during 1996–2005 following the protection of nests with wire cages (61 x 61 x 30.5 cm cages with 5 x 5 cm mesh). The average number of nesting pairs at the site increased from 1.3 pairs/year prior to nest protection to 7.6 pairs/year after protection.

10

A small trial in eastern England (Gulickx et al. 2007) found that neither of two little ringed plover Charadrius dubius nests that were protected by wire cages (61 x 61 x 30.5 cm cages with 5 x 5 cm mesh) in 2005 and 2006 were lost to predation.

A replicated, controlled before-and-after study from 1999-2004 on pastures in southwest Sweden (Pauliny et al. 2008) found that the average hatching rate of southern dunlin Calidris alpina schinzii nests was significantly higher for nests protected by steel cages (20 cm high truncated cones with 7.5 cm gaps between vertical bars and 4 x 4 cm steel mesh covering the top)than for unprotected nests (67% of 25 protected nests survived to hatching vs. 41% of 61 unprotected nests). Moreover, protected nests were more likely to hatch more than one chick (80% of 25 protected nests vs. 57% of 60 unprotected nests). However, comparing 1993-98 (when no nests were protected) with 1999-2004 (when some nests were protected) revealed that there was no significant change in either the number of fledglings/breeding adult or the number of new recruits/breeding adult produced by the study sites. This study is also discussed in ‘Can nest protection increase predation of adults and chicks’?

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Effectiveness

An assessment by independent experts of the effectiveness of this action based on the summarized evidence (0% = not effective, 100% = highly effective). This score is based on the direction and size of the effects reported in each study. Actions with high scores typically have large, desirable effects on the target species/habitat in each study. There is some variation between actions, e.g. 100% effectiveness in adding underpasses under roads for bat conservation will likely have different impacts to 100% effectiveness in restoring marsh habitat. The effectiveness score does not consider the quantity or quality of studies; a single, poorly designed study could generate a high effectiveness score. The effectiveness score is combined with the certainty and harms scores to determine the overall effectiveness category (for more details see https://www.conservationevidence.com/content/page/79).

Harms

An assessment by independent experts of the harms of this action to the target group of species/habitat, based on the summarized evidence (0% = none, 100% = major undesirable effects). Undesirable effects on other groups of species/habitats are not considered in this score. The harms score is combined with the effectiveness and certainty scores to determine the overall effectiveness category (for more details see https://www.conservationevidence.com/content/page/79).

Certainty

An assessment by independent experts of the certainty of the evidence for this action based on the summarized evidence (0% = no evidence, 100% = high quality evidence). How certain can we be that the effectiveness score applies to all targets of the intervention (e.g. all birds for an action in the bird synopsis)? This score is based on the number, quality and coverage (species, habitats, geographical locations) of studies. Actions with high scores are supported by lots of well-designed studies with a broad coverage relative to the scope of the intervention. However, the definition of "lots" and "well-designed" will vary between interventions and synopses depending on the breadth of the subject. The certainty score is combined with the effectiveness and harms scores to determine the overall effectiveness category (for more details see https://www.conservationevidence.com/content/page/79).

Overall Effectiveness Category

The overall effectiveness category is determined using effectiveness, certainty and harms scores generated by a structured assessment process with multiple rounds of anonymous scoring and commenting (a modified Delphi method). In this assessment, independent subject experts (listed for each synopsis) interpret the summarized evidence using standardised instructions. For more details see https://www.conservationevidence.com/content/page/79.